An area covered by a cellular telecommunications network is divided into a plurality of cells. Each of these cells has a base station which is arranged to transmit signals to and receive signals from mobile stations located in the cell associated with the respective base station. Mobile stations will be in active communication with the base station associated with the cell in which the mobile station is located.
In order to improve the capacity and/or the signal to interference ratio, beam steering or beam selection techniques have been proposed. With beam steering or selection techniques, a base station is able to generate one or more beams. In the case of beam selection, the base station will be capable of producing a fixed number of beams to cover the cell associated with the base station. In certain conditions, not all of the beams will be selected for communication with a given mobile station. For example, if it can be determined where the mobile station in question lies in the cell, the beam or beams which cover that part of the cell in which the mobile station is located only will be selected to transmit a signal. Accordingly, that signal for the mobile station in question will not be transmitted in the other non selected beam directions.
Beam steering uses a similar concept. Rather than having a fixed number of beams, the number of beams which are generated can be varied and the width of those beams can be varied. As with the beam selection technique, one or more beams can be generated, in appropriate circumstances, to transmit a signal over only part of the cell.
With both of these techniques, as not all signals are transmitted throughout the whole cell, the interference generated is reduced, which can allow an increase in capacity to be achieved. Both of these techniques use an array of antennas in order to generate the beams.
In addition to transmitting signals which are intended for a given mobile station, the base transceiver station is also arranged to transmit a reference signal which is used by all of the mobile stations in the cell associated with that base transceiver station. Accordingly, this reference signal must be transmitted throughout the cell. In GSM (Global System for Mobile Communications), the reference channel is the BCCH (Broadcast Control Channel).
In GSM, frequency division multiple access is used in combination with time division multiple access. This means that a channel for a mobile station is defined by the carrier frequency used and the associated time slot on that frequency. Accordingly, a base transceiver station will be arranged to use a range of different frequency carriers. Multi carrier power amplifiers are therefore used to drive each element of an antenna array which allows beam steering or selection to be achieved. In order to also provide the BCCH channel, two solutions have been proposed. Firstly, an additional antenna is provided for the BCCH channel which transmits the BCCH channel over the entire cell associated with the base transceiver station. However, this is disadvantageous in that the provision of an additional antenna incurs additional costs.
In the second solution which has been proposed, no additional antennas are provided. Rather, the antennas which provide the beams are also used to transmit the BCCH channel throughout the cell. However, this has the disadvantage that the power requirements for the multi carrier power amplifiers has to be significantly increased to ensure that it is always possible to transmit the BCCH channel throughout the cell. The requirement for the multi carrier power amplifiers to have an increased power requirement increases the costs associated with the power amplifiers.